Sound is heard over longer distances on rainy days because

The speed of sound in a medium is inversely

proportional to the square root of its density.

Higher is the humidity in the air, greater will

be the velocity of sound. Therefore, the speed

of sound in moist air is more than that in dry

air. This is why the sirens of mills, whistle of

trains and others are heard up to longer distances on a rainy day than on a dry day.